The present invention relates to a method and apparatus for copying and manufacturing a hologram plate by a single-beam exposure system.
Conventionally, there are known techniques to effectively mass-produce a number of holograms. The techniques include a method of copying a master hologram and a method of copying an optical element (concave mirror). According to such copying methods, air exists between an optical element (or a master hologram) and a photosensitive substrate (base) for taking a photograph of a hologram. The air is greatly different in index of refraction from a glass of the photosensitive substrate, that is, the index of refraction of the air is 1 and the index of refraction of the glass is 1.52. Thus, such copying methods can cause many unnecessary interface reflections to occur to produce interference fringes. These interference fringes are recorded as noises in the photographing of the hologram, thereby deteriorating the performance of the optical element. For example, in the case of copying a concave mirror, the focal point can be formed at an unnecessary position due to noises having different focal lengths.
One possible solution is that as disclosed in the Japanese Patent provisional Publication No. 60-122983 wherein a refraction index adjusting liquid such as a silicon oil having an index of refraction extremely close to that of the glass is disposed between the photosensitive substrate and the optical element so as to eliminate the interface reflections to prevent the recording of the unnecessary interference fringes. However, there is a problem which arises with such a method in that a large quantity of refraction index adjusting liquid exists between the photosensitive substrate and the optical element because the surface of the optical element facing the photosensitive substrate is arranged to be concaved. In other words, the large quantity of refraction index adjusting liquid exists between two light beams to be interfered. Thus, difficulty is encountered to stably form the interference fringes when the refraction index adjusting liquid moves due to slight vibration, temperature variation, etc. and hence we have to have a long wait until the movement of the refraction index adjusting liquid falls. This causes the cycle time of the exposure process to become extremely long and makes difficult the mass production of the holograms.